A super luminescent diode (SLD) is a light emission element which has characteristics of having a wide light emission spectrum width relatively close to a light emission diode and emitting light at a narrow radiation angle with a strong intensity like a light emission state of a semiconductor laser. The light emission element as described above can be used as a light source of a projector, for example, and is demanded to have a higher luminance.
An SLD described in Patent Literature 1 is provided with a linear ridge waveguide formed perpendicularly to a cleavage end surface and a curved guide active layer provided so as to be curved continuously thereto in a plan view. On the cleavage end surface, an AR (anti-reflection) film is formed in some cases.
In the SLD having such a structure, a most part of light generated in an active layer immediately below the linear ridge waveguide travels toward the curved guide active layer. The light that travels toward the curved guide active layer is divided into light that leaks due to a curve of the layer, light guided to an end surface (end surface opposite to the cleavage end surface) and reflected on the end surface, and light absorbed during being guided.
With this structure, the light that leaks due to the curve and the light that is reflected on the opposite end of the cleavage end surface cannot be returned to the linear active layer, so a laser mode oscillation can be suppressed (see, for example, a lower right column of p. 2 to an upper left column of p. 3 and FIG. 1 in Patent Literature 1).
In brief, unlike a normal laser diode (LD) having a structure in which light is reciprocated and resonated by mirrors provided on both end surfaces, the SLD has a structure in which light is caused to travel on one way by a waveguide and amplified (simulated emission is performed). Those are different in that a spectrum width of a wavelength of output light of the SLD is significantly wider than that of the LD.
Patent Literature 2 discloses a structure of a semiconductor laser (not SLD). In the semiconductor laser, an upper portion of a p-type clad layer as a second clad layer is constituted of a ridge unit and a wing unit, and between the ridge unit and the wing unit, a groove formed from an upper surface of the p-type clad layer to an inner portion thereof is formed.
For a width in a lateral direction (direction orthogonal to a light emission direction) of each of the ridge unit and the groove, a groove width in a rear end surface side area is smaller than a groove width in a front end surface side area. With this structure, a radiation loss of high-order transverse mode light is reduced (see, for example, paragraphs [0036], [0058], and [0077] of description, and FIGS. 1 and 2 in Patent Literature 2).